1. Field of the Invention
The present invention relates to piezoelectric actuators represented by ultrasonic motors and bimorph type actuators used in clocks, cameras, printers, storage devices and the like and, more particularly, to a piezoelectric actuator whose output is improved from that available in the prior art.
2. Description of the Related Art
Piezoelectric actuators that utilize a vibration of a piezoelectric element in response to the application of a driving signal such as an AC voltage as a motive force to move a movable body are attracting attention especially in the field of micromechanics because of their high electromechanical energy conversion efficiency.
A description will now be made which references to FIGS. 13A, 13B and 13C on a piezoelectric actuator 100 which is an example of conventional piezoelectric actuators.
The configuration of the piezoelectric actuator 100 will now be described.
As shown in FIG. 13A, the piezoelectric actuator 100 is substantially comprised of a rectangular elastic plate 101 made of metal, a piezoelectric element 102 integrally stacked on one of the surfaces of the elastic plate 101 and a piezoelectric element 103 formed on the other surface of the elastic plate 101.
The piezoelectric elements 102 and 103 are polarized in the direction of the thickness thereof. Referring to the polarizing direction, for example, surfaces 102a and 103a in contact with the elastic plate 101 are polarized to be negative and positive respectively, whereas surfaces 102b and 103b opposite thereto are polarized to be positive and negative respectively. That is, the piezoelectric elements 102 and 103 are polarized in opposite directions.
An electrode is provided on each of the surfaces 102b and 103b to substantially cover the entire surface. The elastic plate 101 serves as an electrode for the surfaces 102a and 103a. 
An operation of the piezoelectric actuator 100 will now be described.
As shown in FIG. 13A, a voltage is first applied with the electrodes on the surfaces 102b and 103b serving as the negative pole and the elastic plate 101 serving as the positive pole.
The piezoelectric element 102 expands in the longitudinal direction because the voltage is applied in the direction opposite to the polarizing direction of the surfaces 102b and 102a. 
The piezoelectric element 103 contracts in the longitudinal direction because the voltage is applied in the same direction as the polarizing direction of the surfaces 103a and 102b. 
As a result, the piezoelectric actuator 100 is bent in the direction indicated by the arrow X in FIG. 13B, which generates a driving force to move the movable body (not shown) in the bending direction.
When a voltage is applied with the surfaces 102b and 103b as the positive pole and the surfaces 102a and 103a as the negative pole, the piezoelectric actuator 100 is bent in the direction opposite to the arrow X, which generates a driving force to move the movable body in the direction opposite to the direction shown in FIG. 13B.
However, upper limits have existed for the output and displacement of the piezoelectric actuator 100 because it is formed by simply forming one each piezoelectric element 102, 103 on both sides of the elastic plate 101 integrally.
As a technique to improve the piezoelectric actuator 100, a piezoelectric actuator 110 as shown in FIG. 13C has been provided in which piezoelectric elements 104 and 105 identical in configuration to the piezoelectric elements 102 and 103 are formed on the piezoelectric elements 102 and 103, respectively. However, increases in output or displacement was smaller than expected from the magnitude of the voltage, i.e., electric power input thereto. The output or displacement of the piezoelectric actuator 110 could be smaller than the output of the piezoelectric actuator 100 depending on the conditions.
The inventors identified a cause for the above-mentioned problem with the piezoelectric actuator 110 as follows. The same piezoelectric element as the piezoelectric element 102 is used as the piezoelectric element 104, which results in the same amount of expansion in spite of the fact that the expansion of the piezoelectric element 104 must be greater than the expansion of the piezoelectric element 102 because it is located further than the elastic plate 101 having a distortion-neutral plane. The same piezoelectric element as the piezoelectric element 103 is used as the piezoelectric element 105, which results in the same amount of contraction in spite of the fact that the contraction of the piezoelectric element 105 must be greater than the contraction of the piezoelectric element 103 because it is located further than the elastic plate 101.
That is, the piezoelectric element 104 has hindered the expansion of the piezoelectric element 102, and the piezoelectric element 105 has hindered the contraction of the piezoelectric element 103.
The invention has been conceived based on the above-described idea, and it is an object of the invention to provide a piezoelectric actuator which transmits a driving force of a plurality piezoelectric elements to the outside without loss.
In order to solve the above problem, according to one aspect of the invention, there is provided a piezoelectric actuator which is distorted according to an input driving signal to generate a driving force, characterized in that it is formed by integrally stacking a plurality of piezoelectric elements such that they do not hinder the operation of each other.
In the above-described aspect of the invention, for example, the piezoelectric actuator is a bimorph type actuator or an ultrasonic motor.
There is no limitation on the material of the piezoelectric elements.
Further, the thickness of each of the plurality of piezoelectric elements is appropriately adjusted in accordance with the operation and position of the piezoelectric element. Basically, a piezoelectric element is made thinner, the greater the distortion it must undergo. All of the plurality of piezoelectric elements may be different in thickness and, alternatively, some of them may have the same thickness.
In this aspect of the invention, by adjusting the thickness of the piezoelectric elements depending on the operations and positions of the piezoelectric elements, all of the piezoelectric elements contribute to the operation of the piezoelectric actuator without interfering with each other. It is therefore possible to fabricate a piezoelectric actuator which provides output greater than that available in the prior art with the same power consumption, which can be made smaller in size than that in the prior art having the same output and which consumes less power.
According to the invention, there is provided a piezoelectric actuator as described above, characterized in that the thickness of piezoelectric elements located on the side of the actuator with smaller distortion is larger than the thickness of piezoelectric elements located on the side thereof with greater distortion.
In this aspect of the invention, the thickness of each piezoelectric element is smaller, the further the piezoelectric element from a distortion-neutral plane of the actuator. Therefore, all of the piezoelectric elements contribute to the operation of the piezoelectric actuator without interfering with each other. It is therefore possible to fabricate a piezoelectric actuator which provides output greater than that available in the prior art with the same power consumption, which can be made smaller in size than that in the prior art having the same output and which consumes less power.
According to the invention, there is provided a piezoelectric actuator as described above, characterized in that at least two of the plurality of piezoelectric elements undergo identical vibrations.
All of the plurality of piezoelectric elements may undergo identical vibrations.
When the piezoelectric actuator is an ultrasonic motor, for example, the identical vibrations may be longitudinal vibrations, bending vibrations or torsional vibrations.
In this aspect of the invention, since at least part of the plurality of piezoelectric elements undergo identical vibrations, the vibrations are greater in magnitude than those in the prior art. It is therefore possible to fabricate a piezoelectric actuator which provides output greater than that available in the prior art with the same power consumption, which can be made smaller in size than that in the prior art having the same output and which consumes less power.
According to the invention, there is provided a piezoelectric actuator as described above, characterized in that the plurality of piezoelectric elements are stacked in a direction in parallel with a driving force extracting portion of the piezoelectric actuator.
In this aspect of the invention, the same effect as that described above is achieved.
According to the invention, there is provided a piezoelectric actuator as described above, characterized in that it is an ultrasonic motor which utilizes a composite vibration resulting from two different kinds of vibrations generated at the piezoelectric elements as a driving force and in that the two different kinds of vibrations are excited by separate piezoelectric elements.
The two different kinds of vibrations are, for example, a torsional vibration and an expansion vibration, although not limited to them.
Further, there are normally a plurality of piezoelectric elements for exciting each kind of vibration, and the thickness of them is adjusted such that each vibration does not interfere with the vibration, i.e., distortion of other piezoelectric elements.
In this aspect of the invention, in addition to the effect as described above, the adjustment of the thicknesses of the plurality of piezoelectric elements makes it possible to optimize the ratio of the magnitudes of the two different kinds of vibrations.
According to the invention, there is provided a piezoelectric actuator as described above, characterized in that it includes a piezoelectric element for detecting vibrations and in that the piezoelectric element for detecting vibrations is different in thickness from the other piezoelectric elements. Since driving piezoelectric elements are provided in a region which is greatly distorted, the detecting capability is higher, the smaller the thickness of the detecting piezoelectric element which is provided in a region having smaller distortion is.
In this aspect of the invention, the piezoelectric element for detection vibrations does not hinder the distortion of piezoelectric elements used as a source of a driving force and has higher detecting capability. This improves the accuracy of control over a piezoelectric actuator.
According to the invention, there is provided a piezoelectric actuator as described above, characterized in that it is an ultrasonic motor in which the thickness of the plurality of piezoelectric elements is equal to the thickness of a vibrating element integrally stacked on the plurality of piezoelectric elements.
In this aspect of the invention, since a driving force generated at the plurality of piezoelectric elements is transmitted to the vibrating element with highest efficiency, it is possible to fabricate an ultrasonic motor which provides output greater than that available in the prior art with the same power consumption, which can be made smaller in size than that in the prior art having the same output and which consumes less power.
According to the invention, there is provided an electronic apparatus having a piezoelectric actuator as described above.
For example, the electronic apparatus is an electronic clock, measuring apparatus, camera, printer, machine tool, robot, transfer apparatus, storage apparatus or the like.
In this aspect of the invention, an ultrasonic motor as described above is used which provides greater output with low power compared to conventional ultrasonic motors. Since this makes it possible to make an ultrasonic motor compact, an electronic apparatus with an ultrasonic motor can be provided which is compact and consumes less power.